First, read the article on beds and mattresses http://www.sunningdaleosteopaths.co.uk/articles/2009/7/31/beds-and-mattresses.html then, if you think your mattress is too hard, decide if your mattress is a good one, or if it's worn out.
If it's a good one, then buy a mattress topper. It's a lot less expensive than a new mattress, and can make quite fast changes to how you feel.
But if your mattress is in need of replacement, then however good a topper is, it won't make your old mattress any better and you will need to consider a new bed and mattress.
A good topper can make a world of difference, but you need to check first.